Standard for electrical measurements



H. T. WILHELM STANDARD FOR ELECTRICAL MEASREMENTS 2 Sheets-Sheet l Filed DeC. 30, 1943 A TTORN April 1o, 1945. H, T VWLHELM 2,373,156

STANDARD FOR ELECTRICAL MEASUREMENTS FiledDec. 30, 1943 2 Sheets-Sheet 2 PLUG cous/NA r/o/v /'.s/5 #Luc cous/NA r/oN /s (ma) A T TORNE Y Patentedl Apr. 10, 1945 STANDARD FOR ELECTRICAL MEASUREMEN TS Henry T. Wilhelm, Long Island City, N. Y., assignor to Bell `Telephone Laboratories, Incorporated, New York, York N. Y., a corporation of New Application December 30, 1943, Serial No. 516,224

of yielding all of the steps of the arithmetic 1s claims This invention relates to standard impedance boxes and more particularly to adjustable impedance standards of the plub box type.

During recent years technological developments have required that high resistance ,measurements bel made to a much higher degree of precision than heretofore. This trend has established more stringent requirements on the accuracy and precision of the resistance standards used and have also necessitated devising such standards having their resistances somewhat more nearly comparable in magnitude to the values of the resistances being measured. This has presented the inherent problem of securing certification of the accuracy of the high resistance standard. Heretofore it was not uncommon to construct the high resistance standard up to a total of one megohm, usually comprising a plurality of series-connected units which may be strapped out of circuit to secure various steps of resistance. The degree of flexibility as to variety of resistance values with this arrangement is quite limited and greater flexibility is desired, not only to secure a greater variety of standard values but also to achieve a higher degree of accuracy and also to permit the same structure to be employed as an adjustable conductance standard. A very desirable arrangement is to provide an adjustable standard having an arithmetic series of decimal fraction steps from 0.1 to l and the arithmetic whole number series from l to l0, inclusive, which series is available both on a resistance and on a conductance basis. To make such a standard really practicable it is also essential that means be provided for obtaining the various steps conveniently and.

expeditiously. It is also necessary that the construction should be such as to avoid undesirable leakage paths which will effect the accuracy of the standard unit.

Many of the principles employed in constructing a standard for one type of impedance element can very often be employed for construct- `ing other types. For example, capacitance standards can be built employingr substantially the same plug-box construction as that employed for resistance or conductance standards.

Itis an object of this invention to provide an adjustable standard containing only ten equal valued circuit elements and which is capable series from 0.1 to 1 and from 1 to 10, inclusive.

It is a further object of this invention to provide an adjusting means for this standard capable of providing the above stated series of standard values expeditiously and with ease and without introducing undesirable errors due to spurious leakage paths.

A still further object is to provide an adjustable standard having a plurality of standard units which can easily be checked against each other and'which can be calibrated at a lower standard value with an effective improvement in precision.

The foregoing objects are attained by this invention by providing an adjustable standard having a plurality of equal valued standard circuit elements electrically isolated from each other, a pair of main conductors to which they may be connected, a connecting means adapted to connect said elements together and to the main conductors in various series and parallel network combinations, thereby providing a large number of different standard values with a limited number of standard elements.

The invention may be better understood by referring to the accompanying drawings in which:

Fig. l shows a plan view of the panel of a standard made in accordance with this invention;

Fig. 2 is an elevation view of the standard shown in Fig. 1;

Figs. 3 and 4 are two views of a preferred connector especially adapted for deriving the desired networks from the standard of this invention;

Figs. 5 and 6 show two views of a single-prong connector to be used in conjunction with the double-prong connector of Figs. 3 and 4 for providing additional networks;

Fig. 7 is a perspective view showing how the double-prong connectors of Figs. 3 and 4 may be used in combination with the single-prong connector of Figs. 5 and 6;

Fig. 8 shows a special flexible connector for providing additional networks; and

Figs. 9 to 13, inclusive, show various network combinations typical of those provided by the apparatus of this invention.

Referring now more particularly to Fig. 1, in

\ all equal.

which reference numeral I designates a panel of high quality insulating material having mounted thereon four rows of jacks, 2, 3, ll and 5, respectively. The jacks in row 2 are all strapped together by bus bar to terminal E. Jack rows 3 and 4 have connected therebetween a plurality of standard circuit elements 9, each isolated from the other. For illustrative purposes these circuit elements are shown as resistors which are exactly equal to each other to within very close limits. While resistors are used to illustrate the invention it is to be understood that capacitors may also be used. The jacks in row v5` are like those in row 2 in that a bus,

bar 2| interconnects them and ties them to a terminal 'I. nection to ground and is normally connected within the standard enclosure to the shield which is not shown.

It should be noted that the horizontal and vertical distances between adjacent jack centers are This arrangement is preferable in the practice of this invention, as it enables a very rapid interconnection of the various jack rows by double-prong plugs I 0, one of which is shown inserted in the left end of jackrow 5. Of course, the particular position of the double-prong plug I0 as shown in jack row 5 makes no useful connection but in the description toi follow it Will be seen how these plugs are used to form a great variety of useful network combinations. j

Fig. 2 shows the elevation view forthe panel of Fig. 1. Here it will be` seen how the jacks, as for example those of row 5, are assembled. These jacks are commercially available and while they are believed ideal for this purpose.

other jacks providing substantially equivalent features can be substituted without departing from the invention. The particularjack illustrated comprises `a hollow, hexagonallyheaded brass bolt, thehollow part being substantially tubular and is designated by reference numeral Fig. l. In Fig. 2 their end view Shows them to be of the cartridge type but obviously other forms can be used. Where an inequality exists substantially exact equality between these standard elements 9 may be secured by connecting'them to the jacks in rows 3 and l through'padding resistors such as resistor 9A. rFliese padding ren sistors may be capable of somer adjustment in their construction. This technique is, .well known in laboratory practice and requires no further detailed description. As most resistor. materials show a slight increase inresistance due to aging, the padded resistors comprising `re-A sistors 9 and padding resistors 9A are made slightly less than the nominal value. A record padding resistor 9B is used to bring theunit to substantial equality with `thenominal value. This second padding resistor may be removed or replaced after vaging has taken place.

Figs. 3 and -4 show two views of the two-pronged plug I0. Here` it will be seen that the plug parts II and I2 have resilient ends I8 and I9, respectively, and that like the jacks, they have hollow upper portions Il which serve in Terminal 3 is provided fora conturn as jacks for other plugs for more complex plug combinations. The plug parts Il and I2 are secured to a heavy conductor I5 by means of plug into a particular pair of jacks, it is occasionally necessary to connect one of these jacks to j another vertical or horizontal adjacent jack.

This is done by iirst inserting one of the singleprong plugs 2,2 shown in Figs. 5 and 6 into that adjacent jack. Then another double-prong assembly I can be made to complete the connection. This is shown more clearly by the perspective View of Fig. 7 wherein is shown a plug assembly I0 inserted into two adjacent jacks in row 3, thereby connecting two of the resistors 9 together. Then a single-prong plug assembly 22 is inserted into a jack in rowA 4 so that a second double-prong plugassembly I 0 can connect the first one to the jack in row 4.

The above described double and single-prong plugs I0 andv22 together with the jack arrangement and main conductors orbus bars provided all the necessary connections for the complete arithmetic series of decimal fraction and whole valued standard resistances previously men tioned. As will be hereinafter more fully described, this same structure will also provide a complete arithmetic decimal fraction and whole valued series of conductance standards. In ad` dition to these most desirable standard values a very large number of other Values of resistance and-conductance can be provided by using those `very simple single and double-prong plugs.

In addition to the large number of plug combi,- nations which can be derived from the use of the single-prongl and double-prong plugs described above, a large number of additional combinations providing standard resistance and standard conductance values may be obtained by using the exible two-pronged plug assembly shown in Fig. 8. Each of the plug parts 28 of this exible assembly corresponds with the similar parts I I and I 2 .of Fig. 4 and is also hollow to form a jack similar to that shown at I'I in Fig. 3. These flexible plugs differ from the double-prong plugs of Figs. 3 and 4, primarily in that the solid conductor I5, of Figs. 3 and 4 is replaced by a ilexible one 3l as shown in Fig. 8. The insulated flexible conductor 3| passes through holes in spacers 30 which may be of insulating material and the bared ends of this flexible conductor are connected to plugs 28 by inserting th'em in holes and securing them with set Screws 29 in an obviously conventional manner. It is preferable to have spacers 3D made of insulatingmaterial to eliminate the possibility of a short-circuit should two of them from different plug assemblies be yinserted in adjacent jacks on panel I.

Tdble I Plug combination Resistance, megohnis Table r1 Conductanoe, micromhos The symbols used in the foregoing tables are easily followed. Any number appearing in the columns headed plug combination without a superscript like an exponent denotes a network of that number of one-megohm units connected in series. The superscripts denote that number of networks of resistance equal to the base numeral which are connected in parallel (the base number in the tables above have been selected to be always equal to one). The letters S and P denote series and parallel respectively and indicate that the series and parallel networks indicated by their adjacent numbers are to be so connected. A network indicated in parentheses denotes this network should rst be set up before completing the other indicated operations. If no parenthesis appears, it indicates that it is immaterial in what order the networks are set up. By way of example the numeral 3 indicates a series network of three resistors each of one-megohm resistance making a network resistance f three megohms: The numeral 13 denotes that three resistor units each of one-megohm resistance are connected in parallel making a network resistance of onethird megohm. The symbol SPI3 as in Table I denotes that the three series connected resistor units are paralleled with the three parallel connected units making a total network resistance of 0.3 megohm. Taking the 0.6 micromho plug combination in Table II as another example, the network in parentheses should first be set up by connecting two one-megohm resistor units in series, then connecting this series combination in parallel with a one-megohm unit making a network resistance of two-thirds megohm (or conductance of three-havles micromhos). Then this two-thirds micromho network should be connected in series with a one-megolun resistor unit as indicated making a total of one and two-thirds megohms or five-thirds megohm resistance. This, of course, is 0.6 micrornho, the required value. From these examples the other symbols are very easily understood.

To more clearly indicate how these connections are so easily and quickly made with the apparatus of this invention, reference is made to Figs. 9 to 13. inclusive, where it is again assumed for illustrative purposes that each of the resistor units 9 has a resistance of one megchm. This corresponds with the value assumed for the resistor units in Tables I and II. The reference numerals are those previously employed. Fig. 9 shows a simple series connection of three units providing a resistance of three megohms. In Fig. 10 a simple parallel connection of three of the resistor units is shown which provides a. conductance of three micromhos. In Fig. 11 the plug combination provides a resistance of 0.6 megohm. In Fig. 12 the combination necessary to provide a resistance of 0.7 megohm is shown. Fig. A13 shows a plug combination for a conductance of 0.6 micromho.

Some structures 0i the prior art will provide many of these combinations but not all of them with so simple an arrangement of parts. For example, the combinations shown in Figs. 11 and 13 are impossible with any known prior art stand. ard resistor or conductance plug-box structure. By reason of the simple addition of the two bus bars 2li-and 2l, they are both possible with this invention. These bus bars permit the rst series or parallel networks and their series network combinations to be set up on the two middie rows of jacks 3. 4 and their final parallel connections to be made to the bus bars 20, 2| through their jacks in rows 2 and 5. External connections are then made to the bus bar terminals 6 and 'l as previously indicated.

In addition to the most desirable values shown in Tables I and II, a very large number of other valuable combinations are possible. Examples of some of the more interesting of these for resistances are 0.125 megohm derived by plug combination la, 0.25 megohm derived by plug combination i4, 0.3333 megohms by plug combination I3, 0.375 meghom by plug combination 3P3Pl2, 0.6666 meghom by Vcombination 2PI, 0.75 megohm by combination 3PI, 0.875 megohm by combination IPI, and 1.25 megohm by combination (2812)?(2Sl2) or by the more simple combination ISH. From these examples it is obvious how other ones are derived. It will also be apparent that many of the resistance and conductance values can be derived in different Ways from the limited number of ten equal resistors. For the standard in the example above, one megohm can be derived by various plug combinations such at 22, 33, 4P4P2. Also two megohms can be derived from plug combinations @and EP3, and 0.6 megohm may be derived fromvplug combinations 6P2Pl and 3P3P2P2 or 321222. These various combinations,r which are substantial identities and provide equal resistance values, are a ready means for further checking the standard resistor units 9 against themselves to detect errors.

Standards laboratories have been hesitant to certify resistance standards having resistances exceeding one megohm in value. The accuracy certified is better for lower resistance values, lorl example, resistance values of the order of 0.1- meohm or 0.01 mezohm. By the peculiar con` struction afforded by this invention it is possible for high resistance standards thereby securing for the high resistance standard an effective certication substantially equivalent to that secured for the lower resistance ranges. Consequently, where this invention is embodied'in the construction of a high resistance standard exceeding a maximum value in the order of' one megohm is arranged for calibration at a lower resistance value, i It should, of course, be understood `that the practice of this invention is not limited to any particular range of resistance values. example, the standard may have amaximum resistance of only one megohm which may be secured by `combining in series ten ,0.1 megohm resistors so that calibration may be made with the resistors in parallel producing a resistance' of only 0.01 megohm. For these lower resistance standards the same advantages of increased accuracy may be realized. K-

Other important advantages of' constructing a standard inthe manner illustrated by this invention are that by reason' of having the separate resistor elements isolated from each other it is possible to make insulation resistance measurements between the separate elements either to insure that its effect may be neglected or to evaluate its effect if not negligible. This construction also makes it possible to make a comparison of any one resistor element; with any other one in the standard thereby insuring that the individual resistors will remain precisely,

equal to each other within very close limits. By reason of this high degree of equality together with the precise knowledge of the magnitude of the insulation "resistance it is possible to certify the series connection of all of the equal resistors to the same degree of precision as for their parallel connection. The certification of the parallel connection is, therefore, eectively used to certify Y the series connection which is one hundred times the resistance of the parallel connection` where ten resistors are used. This will be recognized by those skilled in this art to be a distinct advantage in the construction, maintenance and certification of resistance and conductance standards.

A preferred method of constructing and calibrating the resistor units `of this invention is to first select as stable resistor material as possible withthe least resistance 'temperature and thermal electromotive force coefficients. resistor units 9 are then made near Vto but slightly less than the required nominal value. Following this they are run through temperature cycles to determine their temperature coefficients and aging characteristics. The relative values of the component resistors are measured at a known temperature so the necessary padding resistors 9A can be connected to make them all equal to withinV very close limits say, for example, twenty parts per million. It should here be stated that it has been found much easier to pad these units to be equal to within very close limits than to try to adjust them alll to be exactly equal to some specified value. The ten padded resistors may For The several then be connected in parallel to form a nominal y resistance of one-tenth the unit value and the absolute resistance determined at the known temperature on a precision Wheatstone bridge to an accuracy of 0.005 per cent or better.' It will thus be seen that the absolute value of each comn ponent resistor unit 9 will be substantially equal tion of one or more of the ten padded resistor units.

As a high degree of precision is increasingly more difficult to achieve for the higher valued resistance standards the practical advantage of this invention is best realized for the resistance standards of higher resistance value. From the foregoing description it is clear that in order to practice the teaching of this invention it is necessary that the resistance standard be constructed with a plurality of resistor elements all of which are isolated from each other, that the individual resistor elements must all be equal to within very close limits and that it must be possible to connect all of the separate resistor units in parallel for calibration purposes. Then in order to derive the benefits of great flexibility as taught by this invention it must be possible for these individual resistor elements to be connected in a great variety of series and parallel networks. This latter feature is provided in a very easy and convenient manner by the structure of this invention.

This invention has been described using resistors as the standard circuit elements. It is obvious that a practical capacitance standard may be constructed in accordance with this invention by merely substituting high quality standard capacitors for the resistors 9. Padding is accomplished by shunting these capacitors with suitable padding capacitors to perform a function analogous to that of the padding resistors 9A. Calibration is effected by connecting the capacitors all in series to secure a low value. Where ten capacitors are used the parallel connection then provides a capacitance one hundred times as large as the series connection but to the same degree of accuracy. It will, therefore, be seen that the same advantages of flexibility in securing large numbers of standard values as well as the advantages of easy use and accuracy are al1 achieved for the capacitance standard in the same way as for the resistance standard.

What is claimed is:

l. An adjustable standard for electrical measurements `comprising a plurality of standard circuit elements electrically isolated from each other, two main conductors isolated from each other and from the standard element, a pair of main terminals, one of the main terminals being permanently connected to one of said main conductors and the other main terminal to the other main conductor, a plurality of connectors for con-, necting said standard circuit elements together in various combinations of series and parallel networks, some of said connectors also connecting said series and parallel networks in series or parallel to said two main conductors, whereby adjustments providing a large number of standard values may be derived from a limited number of standard circuit elements.

2. An adjustable standard for electrical measurements comprising a plurality of equal valued standard circuit elements electrically isolated from each other, twofnain conductors isolated' conductors,l whereby adjustments providingl a large number'ofstandard values may be derived from a limited number of standard circuit elements.

3. An adjustable standard for electrical measurements comprising a plurality of standard circuit elements electrically isolated vfrom each other, two main conductors isolated fromv each other and from the standard elements, a pairV of main terminals one of the main `terminals being permanently connected to one of said main conductors and the other` main terminal to the other main conductor, two connecting terminals for each standard circuit element, a plurality of similar connecting terminals for each of said main conductors, and a plurality of connectors for connecting said connecting terminals together to form various combinations of networks, whereby adjustments providing a large number of standard values may be derived from a limited num ber of standard circuit elements.

4. An adjustable standard for electrical measurements comprising a' plurality of equal valued standard circuit elements electrically isolated from each other, two main conductors isolated from each other and from the standard elements, a. pair of main terminals, one of the main terminals being permanently connected to one of said main conductors and the other main terminal to the other main conductor, two connect-'f ing terminals for each standard circuit element, a plurality of similar connecting terminals for each of said main conductors, and a plurality of connectors for connecting said connecting terminals together to form various combinations of -Y networks, whereby adjustments providing a large number of standard values may be derived from va limited number of standard circuit elements.

5. An adjustable standard for electrical measurements comprising a plurality of standard circuit elements electrically isolated from each other, two main conductors isolated from each other and from the standard elements, a pair of main terminals, one of the main terminals4 being permanently connected to one of said main conductors and the other main terminal to the other main conductor, two connecting terminals for each standard circuit element, a plurality of similar connecting terminals for each of said main conductors, each of said connecting terminals being disposed equidistant from its adjacent connecting terminals, and a plurality of connectors for connecting any of said adjacent connecting terminals together to form various combinations of networks, whereby adjustments providing a large number of standard values may be derived from a limited number of standard cir cuit elements.

6. An adjustable standard for electrical measurement comprising a plurality of equal valued standard circuit elements electrically isolated from each other, two main conductors isolated from each other and from the standard elements, a pair of main terminals, one of the main terminals being permanently connected to one of said main conductors and the other main terminal to the other main conductor, two connecting terminals for each standard circuit element, a plurality of similar connecting terminals for each of said main conductors, each of said connecting terminals being disposed equidistant from its adjacent connecting terminals, and a plurality of connectors for connecting any of said adjacent connecting terminals together to form various combinations of networks, whereby adjustments providing a, large number. of standard values may be derived from a limited number of standard circuit elements.

'7. A combined adjustable resistance and conductance standard for electrical measurements comprising aplurality of standard resistance elements electrically isolated from each other, two main conductors isolated from each other and from the standard elements, a pair of main terminals one of the main terminals being permanently connected to one of said main conductors and the other main terminal to the other main conductor, a plurality of connectors for connecting said resistance elements together in various combinations of series and parallel networks, some of said connectors also for connecting said series and parallel networks in series or parallel to said two main conductors, whereby adjustments providing a large number of standard resistance' and conductance values may be derived from a limited number of standard elements.

' 8. A combined adjustable resistance and conductance standard for electrical measurements comprising a plurality of equal valued standard resistance elements electrically isolated from each other, two main' conductors isolated from each other and from the standard elements, a pair of main terminals one of the main terminals being permanently connected to one of said main oo nductors and the other main terminal to the other main conductor, a plurality of connectors for connecting said resistance elements together in various combinations of series and parallel networks, some of said connectors also for connecting said series and parallel networks in series or parallel to said two main conductors, whereby adjustments providing a large number of standard resistance and conductance values may be derived from a limited number of standard elements.

9. A combined adjustable resistance and conductance standard for electrical measurements comprising a plurality of standard resistance elements electrically isolated from each other, two main conductors isolated from each other and from the standard elements, a pair of main terminals one of the main terminals being permanently connected to one of said main conductors and the other main terminal to the other main' conductor, two connecting terminals for each resistor, a plurality of similar connecting terminals for each of said main conductors, and a plurality of connectors for connecting said connecting terminals together to form various combinations of networks, whereby adjustments providing a large number of standard resistance and conductance values may be derived from a limited number 'of standard elements.

10. A combined adjustable resistance and conductance standard for electrical measurements comprising a plurality of equal valued standard resistance elements electrically isolated from each other, two main conductors isolated from each other and -from the standard elements, a pair of main terminals one of the main terminals being permanently connected to one of said main conductors and the other main terminal to the other main conductor, two connecting terminals for each resistor, a plurality of similar connecting terminals for each of said main conductors, and a plurality of connectors for connecting said connecting terminals together to form various combinations of networks, whereby adjustments providing a large number of standard resistance and conductance values may be derived from a limited number of standard elements` l 11. A combined adjustable resistance and conductance standard for electrical measurements comprising a plurality of standard resistance elements electrically isolated from eachother, two main conductors isolated from each other and from the standard elements, a pair of main terminals one of the main terminals being permanently connected to one of said main conductors and the other main terminal to the other main conductor, two connecting terminals for each resistor, a plurality of similar connecting terminals for each of said main conductors, each of said connecting terminals being disposed equidistant from its adjacent connecting terminals, and a plurality of connectors for connecting any of said adjacent connecting terminals together to form various combinations of networks, whereby adjustments providing a large number of standard resistance and conductance Values may be-derived from a limited number of standard elements.

12. A combined adjustable resistance and conductance standard for electrical measurements comprising a plurality of equal valued standard resistance elements electrically isolated from each other, two main conductors isolated from each other and from the standard elements, a pair of main terminals one of the main terminals being permanently connected to one of said main conductors and the other main terminal to the other main conductor, two connecting terminals for each resistor, a plurality of similar connecting terminals for each o-f said main conductors, each of said connecting terminals being disposed equidistant from its adjacent connecting terminals, and a plurality of connectors for connecting any of said adjacent connecting terminals together to form Various combinations of networks, whereby adjustments providing a large number of standard resistance and conductance values may be derived from a limited number of standard elements.

13. The combination in accordance lwith claim 1 wherein the standard circuit elements comprise standard electrical capacitors.

Y 14. The combination in accordance `with claim 2 wherein the standard circuit elements comprise standard electrical capacitors. ,j s

l5. The combination in accordance with claim 3 wherein the standard circuit elements comprise standard electrical capacitors.

16. The combination in accordance with claim 4 wherein the standard circuit elements comprise standard electrical capacitors.

17. The combination in accordance with claim 5, wherein the standard circuit elements comprise standard electrical capacitors.

18. The combination in accordance with claim 6 wherein the standard circuit elements comprise HENRY T. WILHELM. 

